Ian Miller SAAB Variable Compression Motor Abstract Ever more stringent emissions standards and the depleting of fossil fuels, supply the need for highly efficient modes of transportation. Technologies such as electric and fuel cell vehicles hold the most promise for emissions and fossil fuel use, however they do not suit the intact infrastructure for automobile fueling. Therefore there is a need for developing highly efficient Otto cycle engines for use in personal transportation vehicles. One way to reduce fuel consumption and thereby CO2 emissions, is to substitute large displacement engines with smaller engines that are equal in torque and power output. Apart from the smaller engine’s reduced friction and weight, it also operates at a specific higher load resulting in improved efficiency. Introduction Modern turbocharged engines like the Saab 2.3L Aero, already represents a certain amount of downsizing, compared to a corresponding naturally aspirated engine in terms of power and torque. The 5 cylinder 1.6L Saab Variable Compression (SVC) concept engine’s performance of 225Hp brings the downsizing concept further, by effectively replacing a 3.0L naturally aspirated engine. To reach high specific power and torque, a combination of variable compression ratio and high pressure charging is used. Downsizing then transforms the high specific performance reached into good fuel consumption, but with performance equal to a larger engine.
If the engine is intended to replace a larger engine in a medium or large size car not only the maximum achieved power and torque output are of vital importance, but also the torque at 1000 RPM and below. To meet these objectives, the SVC engine uses a mechanical supercharger instead of a turbocharger. The high Indicated Mean Effective Pressure (IMEP) necessary for the SVC 1.6L engine to equal the 3.0L NA engines performance forces the CR down to 8:1 to avoid knock. At part load operation where most of the driving occurs, a fixed CR of 8:1 would have a considerable negative impact on overall fuel consumption. The SVC engines
Variable Compression Ratio (VCR) system enables CR 14:1 to be used during part load operation, thereby achieving good efficiency.
2. Description of the SVC engine
Fig. 2: SVC engine main data (Memmer) When designing the SVC engine the Saab’s engineer’s main objectives for the VCR system were: 1.) The VCR system is not allowed to add any kind of friction to the engine under operation that increases parasitic losses. 2.) The VCR system must not affect the combustion chamber in a negative way, detoriating engine efficiency. 3.) The VCR system should be infinitely variable during operation between CR 8:1 and 14:1. When packaging constraints, functionality etc. were added to the main objectives, the “Split engine block / Pivot type” came out as the most appropriate design. The SVC engine’s cylinder block is divided into two parts. The lower part is the crankcase, where crankshaft, con-rods, pistons, etc. are mounted. The upper part is what Saab calls the “Monohead”, is where the combustion chamber, valves, cylinder liners etc. are situated. The Monohead can be tilted relative to the crankcase through a pivot shaft. The pivot shaft location is a compromise between avoiding too much cylinder axial variation and keeping the engine width down. Tilting of the Monohead (enabling the variation of the CR) is controlled by a device Saab calls the excenter shaft mechanism. The excenter shaft is turned to the chosen position by a hydraulic actuator situated at the front of the engine. A sensor indicates the position of the excenter shaft and thereby the actual CR. A separate hydraulic system supplies hydraulic pressure between 60 - 100 bars to the hydraulic actuator.
Fig. 3: An Excenter shaft mechanism controls the CR (Memmer)
2.1 Monohead Due to the split engine block design, it was possible to make a design in which the cylinderhead with valves, combustion chamber etc, forms one unit together with the cylinder liners,(i.e. a Monohead design). It is not necessary to use a Monohead design, but there are several atvantages, including: More flexible cylinderhead layout due to the lack of cylinderhead bolts. This resulted in increased cooling at critical areas and a small valve angle, beneficial for achieving high CR. No cylinderhead gasket. No cylinder deformation during assembly of cylinderhead bolts, i.e. less piston group friction.
Drawbacks of the Monohead design include: Complex casting, machining and assembly. Limited serviceability.
As the position of the camshaft chain gears relative to the crankshaft changes for each CR, a special layout for the timing chain drive needed to be developed. At the pivot point only rotating movement occurs between crankcase and monohead. Therefore the chaindrive is divided in two separate chains, one primary drive between crankshaft and pivot point and one secondary drive between Pivot point and camshafts. Due to this design, no variation in gearwheel distance occurs
when the CR is changed. The Monohead tilts 4.10 Degrees when the CR is changed between 8:1 and 14:1, this means that the Camshaft rotates 2.05 Degrees relative to the crankshaft. But at the same time, the TDC position changes due to the tilting of the cylinders. In all, the deviation between crankshaft and camshafts is 1.02 Degree advanced when the CR is changed from 14:1 to 8:1.
Fig. 4: Timing Chain Drive Layout.
2.2 Crankcase / Monohead Rubber Seal As the monohead moves relative to the crankcase, flexible rubber bellows are used to prevent crankcase gases and oil from escaping from the engine. To protect the sealing from heat and mechanical damage, shields cover the sealing around the engine. The exhaust ports are also positioned higher than normal, allowing for extra space between exhaust manifold and sealing, keeping the sealing temperature under 120C.
2.3 Charging System With the VCR system, the amount of downsizing that can be achieved is dependent on the charging system performance. Many charging systems were investigated and the only available system capable of delivering the required level of boost pressure over the entire engine speed range was judged to be the “Screw type” supercharger.
Fig. 5: The Lysholm “Screw type” Supercharger, positive displacement, charging process.
Due to the inbuilt pressure ratio in the supercharger of approx. 2:1, it is necessary to disconnect the supercharger during part load operation to avoid parasitic losses. At part load the supercharger is disconnected with help of a clutch, and a “by-pass” throttle is opened, allowing the inlet air to by-pass the supercharger. The engine then operates as a 1.6L naturally aspirated
engine. During high load conditions, the clutch is closed and the SC is engaged. The “By-pass” throttle closes and the boost pressure increases. (Fig. 6) Fig. 6: Schematic layout of the Charging system 2.4 Engine Management System In order to control the added functionality of VCR and the described charging system, a modified Saab Trionic 7 engine management system is used. The system has a VCR map for each load / rpm and new functionality that uses the dual throttles and the supercharger clutch to control the boost pressure. To achieve sufficient injector range from idle to full load for a 1.6L engine with 225Hp, variable fuel pressure is used. The fuel pressure increases with increased boost pressure. 3. Performance 3.1 Power, Torque and BMEP As pointed out already, the amount of downsizing that can be achieved is very much dependent on the available torque at 1000rpm and below. Therefore, the original objective was to create a relatively “flat” torque curve.
Fig. 7: SVC - engine power and torque curve
Fig. 8: BMEP (RON 98) compared to current supercharged and N/A engines To obtain this output from only 1.6L displacement, it has to operate at high IMEP/BMEP. To avoid engine knocking and detoriated combustion stability caused by late phased combustion, the CR has to be lowered down to 8:1 at full load operation. To meet the BMEP objectives, the boost pressure has to be accordingly high over the engines entire speed range. Fig.9 shows a comparison of static boost pressure for a turbocharged Saab 9-5 Aero, and the SVC engine with SC. The better dynamic behavior of the SC means that the difference at low rpm is even greater in reality. The built in pressure ratio in the SC of approx. 2:1, contributes to the good adiabatic efficiency at high boost pressures, which allows the SC to operate within the temperature limits of 185 C at the outlet port.
Fig. 9: Boostpressure comparison Due to the exit temperatures from the SC of up to 185 C, an effective intercooler system is required both in terms of a low-pressure drop, and temperature efficiency. To reach the performance targets, a maximum air temperature of 60 C after the intercooler is required.
3.2 Friction and FMEP Replacing a larger displacement engine with a smaller engine gives an obvious advantage in terms of reduced friction. However due to the SVC engines high peak cylinder pressure, the bearings, pistons etc. have to withstand the correspondingly higher load. This means that FMEP will not be in class with the best NA engines, but real friction power will still be lower than best in class NA engines with the same performance. Due to the large increase in friction power when the SC is used, the strategy is to run the engine as much as possible without SC. 3.3 Emissions One of the main objectives for the SVC engine was that it should be able to meet all known future worldwide stringent emission legislations. To meet this objective, a Lambda = 1 concept with a TWC system was selected. There are also no restrictions regarding further add on systems like EGR, SAI etc. The SVC engines small individual cylinder displacement of 0.32L together with the engine operating at CR 14:1 during part load, puts the focus on HC emissions. In Fig.10 the difference in HC emissions for CR=14:1 and the more normal 10:1 is shown. Due to downsizing, the engine operates at a higher load, which partly compensates for this increase in BSHC. However, it is still necessary to operate the engine at low CR before Catalytic converter “Lights off”. This also reduces “Light off” time, due to the decreased engine efficiency.
Fig. 10: Brake Specific Hydrocarbon output at 2000 rpm, BMEP = 2.0 bar.
3.4 Driveability The SC is disconnected during part load operation to avoid parasitic losses. The connection and disconnection of the SC is operated by an electromagnetic clutch. Due to the moment of inertia from the SC when it is engaged, a control strategy was developed to make the SC switching transparent to the driver. 7
Depending on driving conditions, different engagement times are used to engage the SC. At constant speed driving without the SC, when a small increase in load requires the SC to be engaged, a smooth connection of the SC is necessary to avoid a jerk in the movement of the car, and thus an engagement time of up to 650ms is used. If on the other hand the driver suddenly requires full torque, a quick engagement of the SC is necessary to obtain instant acceleration, and thus an engagement time of less than 100ms is used. Above 3500 rpm, the SC is engaged all the time.
4. Summary and Discussion It has been shown that VCR interacting with High charging and downsizing can reduce drivecycle fuel consumption up to 30%, a NA 3.0L engine can effectively be replaced with a Supercharged 1.6L engine. To equal the performance of the 3.0L engine, a BMEP of over 24 bar was obtained. A Screw type Supercharger was used to create sufficient boost pressure over the engines entire speed range. The Superchargers built in pressure ratio contributes to good efficiency at high boost pressure but causes excessive losses at part load when no boost pressure is needed. Therefore, a clutch disengages the Supercharger at part load. In all, this concept has turned around the Otto engines characteristic of increased efficiency with increased load, to reach maximum efficiency at medium loads. As this is the area where most driving occurs, a real fuel consumption benefit for the customer can be expected. The current limit for the possible downsizing with this concept depends on the charging system, both in terms of possible boost pressure over the entire speed range, and the fact that too much Supercharger operation detoriates fuel consumption. Future charging systems like Electric assisted Turbochargers, Pressure Wave Superchargers etc. will move this limit, making even further downsizing possible. One consideration, which has not been made, is the cost of production for this motor. Ordinarily technologies such as supercharging and intercooling are considered too expensive for most production cars. On top of that there is the expense of the monohead and VCR technology, not only is there an added cost to production for these technologies but in training of mechanics to service them. In the end though the cost of fuel and government regulations will force new and expensive engine technologies into the auto market.
Aki, Saab Corporation, 358-9-48212 x3286, Helsinki Finland,Telephone Interview and e-mail, May 7, 2001, Regarding the Saab Variable Compression Ratio Engine. Allison, L. “Engine Trends.” Internet. 4 May 2001. Available http://www.4wdonline.com/A/Engines.html Kobe, Gerry. “Saab unveils Radical Variable-Compression Engine.” Internet. 4 May 2001. Available http://www.findarticles.com/cf_0/m3012/4_180/61892620/print.jhtml Lee, David. “New Unique Engine Concept for High Performance and Lower Fuel Consumption: Saab Variable Compression.” Internet. 4 May 2001. Available http://www.autoworld.com/news/saab/saab_variable_compression.asp Memmer, Scott. Romans, Brent. “Saab’s Variable Compression Engine.” Internet. 4 April 2001. Available http://www.edmunds.com/edweb/editorial/innovations/saabengine.html. “Saab Reveals Unique Engine Concept That Offers High Performance and Low Fuel Consumption.” Internet. 4 May 2001. Available http://www.saabnet.com/tsn/press/000318.html. Sharke, Paul. “Otto or Not, Here it comes.” Internet. 4 April 2001. Available http://www.mcmagazinc.org/backissues/junc00/features/otto/otto.html. Wan, Mark. “Power Boosting Technology.” Internet. 4 May 2001. Available http://www.fortunecity.com/silverstone/lacia/58/technical_school/engine/tech_engine_4.htm.
Calculations: On the Following pages I calculated the theoretical power output for a 1.6 liter supercharged motor with a compression ratio of 8:1, and a 1.6 liter naturally aspirated motor with compression ratio 14:1, using the stated assumptions. While these calculation neglect the friction associated with the motor and in now way reflect the engine output power, we can conclude the supercharged motor will produce more power than the naturally aspirated one.